This article describes why Outlook does not support read-only media.

There are three reasons why Outlook does not support opening personal folders (.pst) files on read-only media (such as, CD-ROMs) or read-only folders:

•	The .pst file is a database file format. As the user navigates through the Outlook user interface, the user expects the computer to respond with searches and sorts (similar to when a user clicks a view column header to sort the content in the view). The computer must be able to write to the .pst file so that those search and sort procedures are available.
•	The user expects to be able to change the view contents, for instance the user may want to add columns, and the user may want Outlook to remember the current sort order, filters, and so on. The view definition saves the information into a special area in that Outlook folder, therefore the view definition is written to the .pst file.
•	The .pst file is in a single-user file format. The computer uses a special area of the file to write information about the computer that opened the file. The file allows other processes on the same computer to access the file while it prevents other computers from accessing the file until the first computer closes the file. Without this "file lock" feature, the computer is open to file damage when two computers open the .pst file at the same time.

These design decisions reflect the core architecture of Outlook and Messaging Application Programming Interface (MAPI).

Personal folders files are not supported on file shares. For additional information, click the following article number to view the article in the Microsoft Knowledge Base: